Moving platforms include either vehicles such as aircraft, ground vehicles, boats, and spacecraft, or equipment, such as directional antennas, cameras, or turrets, that can be mounted to vehicles and reoriented relative to the vehicle body. The platforms may be traveling at fast or slow speeds, may be maneuvering or non-maneuvering, and may be occasionally stationary relative to geodetic space for time periods of arbitrary lengths. These platforms require knowledge of their geodetic attitude in order, for example, to support safety or stability control systems, to point an antenna, camera, or other sensor boresight at a geodetically known target, to control their geodetic position or attitude movement, or to register the information sensed along the boresight onto a map projection with geodetic coordinates. For the examples, the sensor, camera, or antenna boresight is the centerline of some signal collection or signal transmission aperture.
Earth-rate sensing through gyrocompassing, interferometry using Global Navigation Satellite System (GNSS), such as Global Positioning System (GPS), and transfer alignment (TA) are possible implementation approaches for precision geodetic attitude measurement systems for moving platforms. Each technique is in widespread use with a broad range of specific implementation details, and each technique alone without specific system support has significant limitations for precision pointing.
Earth rate sensing requires the use of a gyroscope with accuracy much better than the earth's 15-deg/hr-rotation rate. The gyroscopes used for conventional
Earth rate sensing requires the use of a gyroscope with accuracy much better than the earth's 15-deg/hr-rotation rate. The gyroscopes used for conventional gyrocompass systems have drift specifications of typically less than 0.1 deg/hr, although some poorer performing systems use gyroscopes with drift specifications of less than 1 deg/hr. Modern military gyroscopes, currently used on missiles, can achieve 1 deg/hr accuracy with prices of about $5000 in large quantities. For a 1 deg/hr tactical weapon grade gyroscope, the north seeking accuracy is about 4 deg and is not sufficiently accurate for high-speed data collection and data access applications.
GNSS interferometry measures GNSS carrier phase to GNSS satellites from multiple spaced antennas. Carrier phase differencing removes all common mode ionospheric corruption from the differenced signals. The remaining phase difference can be used to infer range to GNSS satellites to millimeter (mm) accuracy, if numerous system level error sources are mitigated. The measurement is corrupted by cable-induced phase differences, on-vehicle multipath phenomena, the variation over satellite look angles of the phase delay differences between 2 antennas, and whole-cycle GNSS wavelength ambiguity that is 19 cm for commercial GNSS. A method not based on interferometry is often used to get close to the correct attitude and reduce whole-cycle ambiguity. Commercial motion characterization systems that use GNSS interferometry are available, but impose installation difficulties by requiring multiple antennas dispersed over several square meters. Also, the lack of wide-bandwidth attitude memory prevents any accuracy enhancement through data averaging unless the system is perfectly stationary. For effective operation, GNSS Interferometry requires knowledge of the relative location and orientation of system components including each GNSS antenna and all inertial sensing devices.
Transfer alignment (TA) is the most widely used precision orientation measurement method for military applications, and generically applies to a host of commercial systems. Transfer alignment synergistically combines an Inertial Navigation System (INS) with single-antenna GNSS system to estimate position and attitude. The INS, traditionally used only in military applications and high-end aircraft, has an Inertial Measurement Unit (IMU), which is an internal instrument suite that generally provides calibrated and compensated measurements of three axes of acceleration and three axes of rotation rate measurement. Mathematical manipulation of the acceleration and rotation rate measurements provides the position, velocity, and attitude of the platform at a high bandwidth. However, the navigation solution will drift unless some external corrections are incorporated. For low cost inertial components, the drift will occur rapidly. GNSS external measurement is most often used for the INS corrections of the IMU measurements. For GNSS transfer alignment, INS-derived velocity and GNSS-derived velocity are differenced, and the time-history of the differences is then used to infer errors in assumed geodetic alignment of the INS axes.